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I have SP500 18x11 in the rear and I've gone through 2 sets of tires in about 5 months. The tires wear out prematurely on the inside of the tires exposing the cords. What alignment should I have or is there another possible problem? I had the previous alignment set as close to zero as possible in the rear and stock in the front.

Thanks
 

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First. Don't just try to have them zero everything out. That isn't always the best way to get maximum traction unless you never turn and you never move your car.

Stock alignment or lance alignment. Search em
 

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Inside wear on the rear tires is generally due to the tires being toed in. This can either be due to alignment set statically to positive toe (toe in) or due to worn suspension bushings which allow the wheels to go toe in under load (dynamic toe). Contrary to popular belief, inside wear on the rear tires is due to too much positive toe (toe in). Inside wear on the fronts is due to too much negative toe. Tire wear comes from the axis of rotation not being aligned with the load. On rear wheel drive vehicles for rear tires this is against the propulsion force, for front tires it is resistance against road and bearing friction. Essentially, if the tire is not perfectly aligned with the load, on every rotation the tires will be scrubbed sideways. Only 1/8 inch (3 mm) of toe misalignment front or rear will scrub the tires sideways 28 feet for every mile traveled. If your bushings are worn causing your wheels to go toe in under load, the solution is to set the static toe to the max spec for toe out. Either that or replace the bushings. BTW, alignment machines are notoriously inaccurate on the toe measurement. Very small errors in the toe setting can have a huge impact on tire wear, whereby caster and camber errors have minimal impact on wear (unless grossly out).
 

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The Supra squats a lot under acceleration increasing toe out when you are stressing the tires the most.

You need a little to in.

Later, Steve
 

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Steve, most of the time I agree with you but not this time. The acceleration squat will cause the rear wheels to "toe in" not toe out. If you draw out the force diagram on paper, it is obvious. The physics as I see them are as follows: On acceleration, vehicle inertia resists the forward motion, compressing the bushings so that the car/chassis moves rearward vs. the wheels which results in more toe in. (ie. If you start out at zero toe, it moves to toe in) Due to this "dynamic" toe movement, the insides of the tires are loaded more than outside so the scrubbing force causes eccentric wear on the inside. If you wish to prevent inside wear on the rear tires of a RWD vehicle you need a little toe out so that it goes neutral on acceleration/cruise. It is the same principle for alignment of front wheels on a FWD vehicle. If you look up the toe alignment spec for just about any FWD vehicle, it is negative (toe out) to compensate for this dynamic force tendency. Front wheels on a RWD car normally have a positive toe setting to account for dynamic loading either during cruise (bearing friction and rolling resistance) + braking forces which tend to make the wheels go towards toe out. Ideally you would want all wheels to be at zero degrees toe while cruising, accelerating and braking but due to chassis component flex and bushing deformation under load it is not possible so alignment specs are designed to be a reasonable compromise. Unfortunately, the toe adjustment (compromise setting) shifts as chassis components wear.
 

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normore,

I may be wrong, but let me clarify.

The stock rear Toyota alignment is listed below.

Camber -1.50 +/- 0.75
Toe in (total) 0.120" (3mm) +/- 0.080" (2mm)

As you can see the stock setting is for toe-in.

I believe the stock rear suspension geometery will cause the wheels to toe-out from their static toe-in position. I need to refer to my manual, but typically the Supra (and most of the Japanese supercars) had some rear steer geometery built in. As you accelerate out of a turn, the rear wheels will toe-out allowing better rotation of the car.

Just from experience. When my car was BPU+ my alignment allowed for very even rear tire wear. When I went to a single turbo, all of the sudden my inner rear tires wore out quickly. I added a little more static toe-in and the inner wear was corrected.

Just a quick point. If the outer edge of your tires are wearing, you typically have too much toe-in. If the inner edge of your tires are wearing too much, you typically have too much toe-out.

Sooooo regardless of whether you're getting bushing deflection or the movement is caused by suspension geometery, if you have too much inner edge tire wear than you you have too much toe-out and need to add more toe-in.

Later, Steve
 

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normore,

I see the confusion. My point is the rear suspension squating causes toe-out due to the suspension geometery. You are correct about the force, but that is handled with suspension geometry. I doubt you would have the same rear alignment for a front drive car and rear drive car simply due to the additional squat caused by the rwd.

Later, Steve
 

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normore,

Just a quick point. If the outer edge of your tires are wearing, you typically have too much toe-in. If the inner edge of your tires are wearing too much, you typically have too much toe-out.

Sooooo regardless of whether you're getting bushing deflection or the movement is caused by suspension geometery, if you have too much inner edge tire wear than you you have too much toe-out and need to add more toe-in.

Later, Steve
Steve, I cannot speak for the effect of suspension geometry on toe under load. That would require more in-depth engineering analysis and calculation than I have time or it's worth. However, just looking at the suspension links in the 1997 repair manual (page 9 of the on-line suspension manual) it would appear to me that as the load shifts rearward the effect will be to increase toe-in. Suspension squat effect (ride height) is more difficult to visualize from the diagram but that may have some effect as well.
On the topic of wear and what causes what: The effect of too much positive toe (toe-in) is, as you say, increased outside wear but this only applies to the front wheels of a RWD car. The effect of too much toe in on the rear wheels will be increased wear on the inside edge of the tires. Wear occurs in the area of load concentration which does most of the scrubbing. In the case of the front tires this is the outside edge because the fortce concentration is on the outside due to rolling resistance. In the case of the rear wheels, the wear is on the inside edge because the load concentration is there due to the thrust load on the tire. There are actually two forces acting on the rear tire; thrust and rolling resistance. The thrust load is opposite in direction to the rolling resistance and higher in magnitude so it dictates the force concentration and thus the wear location. BTW, it's not all just theory. I've had lots of isssues in the past with excess tire wear (front and rear) and sorted them out by doing my own manual alignment. For manual alignment I have used a similar method as Ben Lew documented on MKIV.com. I actually modified it to a hybrid system (laser alignment combined with manual) to simplify. The Supra and most sports cars with stiff low profile tires, combined with high HP and agressive driving is very sensitive to toe mis-alignment but realitively insensitive to errors on caster and camber as pertains to tire wear. Coincidently laser alignment machines do a pretty good job on measuring caster and camber (relative to the spec) but are fairly inaccurate on toe relative to the spec. My method is to get a laser alignment, then measure toe manually using tape and string and then make minor adjustments to toe to get it correct. In between adjustments I use this device:
http://www.gunson.co.uk/item.aspx?cat=676&item=1812
to check the toe if I suspect it may have drifted or if I see any abnormal tire wear. It's simple and works.

I know the toe-in causing insde wear on the rear tires is counter-intuititive but it fits with the physics and what I have seen in practice.
 

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nomore,

Interesting that my rear inner tire wear improved when I added some toe-in.

I really don't have time to figure out how your experience and mine differ, so stop confusing me. :)

Later, Steve
 

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Steve, I also have no idea why our experience differs. BTW, I start out each day confused. My apologies if it is catching;)

Supra rear tire inside edge wear is nothing compared to the NSX which is legendary for eating tires fron and rear. Typical tire life was 4-10K miles with factory settings. The NSX came with its suspension alignment set up for maximum handling. The front was toed out and the rears toed in. This apparently was done to improve turn in (front toe out) and stability on the straights (rear toe in). The factory settings were:
Factory front alignment (ALL years):
Total toe: -3.5mm +/- 1mm
Factory rear alignment (ALL years):
Total toe: +6mm +/- 1mm

As you can see the rears were toed in substantially (6mm) which contributed to the fast inside tread wear. There was a class action lawsuit on this. NSX owners learned that if they wanted tires to last that had to give up on the factory ultimate handling alignment configuration and typically reduced the toe out on the front and toe in on the rears to close to neutral (0.0mm).
 
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